The invention relates to high intensity discharge (HID) lamps, and more particularly to an electrodeless HID lamp employing a high-temperature excitation coil.
In an electrodeless HID lamp, a light emitting ring-shaped arc discharge is induced in a gas-containing or plasma-containing arc tube by a radio-frequency electric current in an excitation coil surrounding the tube. High temperatures (above 1000.degree. C.) are required in the arc tube to prevent the gas from condensing, yet the coil must not be subjected to temperatures that approach its melting point. HID lamp induction coils of prior art design, typically made of copper, must not be exposed to temperatures of more than approximately 200.degree. C. above room temperature to prevent excessive resistive losses in the coil and to prevent coil oxidation in the ambient air. This is accomplished by coil cooling. Home, the cooling requirement is difficult to meet in a commercial lamp that must be limited in cost, size and power input. A cool coil also requires adequate insulation between the arc tube and the coil; otherwise, the heat load on the coil can become excessive, and the temperature of the arc tube is likely to fall below approximately 1000.degree. C. with attendant condensation of the vapors in the arc tube. Therefore, the induction coil of prior art HID lamps is located external to the lamp envelope, and the envelope is separated from the arc tube by a layer of insulation. These intervening insulating layers lead to effective coil diameters that are much larger than the arc diameter, causing poor coupling and high coil currents which result in high power losses in the coil and in the power supply ballast.